1. Technical Field
The invention relates to apparatus for converting optical signals and especially to optical apparatus for use in optical communications systems in which optical signals of different wavelength propagate in opposite directions in a single optical transmission path. The invention is especially applicable to optical apparatus for use with optical communications systems employing wavelength-division multiplexed (WDM) optical signals.
2. Background Art
In optical communications systems, a single optical transmission path, in a suitable transmission medium such as an optical fiber, may convey bidirectional optical signals, i.e. a first optical signal of one wavelength in one direction and a second optical signal of a second wavelength in the opposite direction. In certain situations, it is desirable to treat the bidirectionally propagating signals by means of a device through which the signals must pass unidirectionally from an input to an output. This involves interrupting the transmission path, routing the two signals unidirectionally through the optical treatment device, and then returning them to the transmission path to continue in their original, opposite directions of propagation. For example, where optical signals are propagating in a long transmission line, it may be necessary to amplify the bidirectional signals at an intermediate point. Converting them into unidirectional signals temporarily enables one, unidirectional amplifier to be used, saving expense and avoiding differences of amplification.
It is possible to insert into an optical transmission path a four-port optical device comprising a pair of one quarter pitch graded index rod lenses positioned coaxially end-to-end with a bandpass filter sandwiched between them. A first port and a second port are positioned at the exposed end of one of the lenses and off-center with respect to the axis of the lens. A third port and a fourth port are positioned at the exposed end of the other lens and correspondingly off-center so that a light beam entering one of the ports will be expanded by the rod lens and collimated as it reaches the filter. If transmitted by the filter, the light beam will be refocussed by the other lens to couple to the opposite port. If reflected by the filter, the light beam will be collimated by the same rod lens and coupled to the adjacent port.
The respective ends of the transmission medium are connected to the first and third ports and the amplifier or other equipment is connected between the second and fourth ports. Signals having a wavelength within the passband of the bandpass filter entering one port pass through the filter to emerge from an opposite port. Signals of the other wavelength, however, are reflected by the filter to emerge from the adjacent port. Hence, a first signal having a wavelength outside the passband, arriving at the first port will be reflected to emerge via the second port and pass through the amplifier or other device. A second signal having a wavelength within the pass band of the filter travelling in the opposite direction in the transmission medium will enter the device via the third port, be transmitted by the bandpass filter, and also emerge via the second port. Consequently, both signals will pass through the amplifier in same direction.
On leaving the amplifier, the two signals will enter the device via the fourth port. The first signal will be reflected again by the bandpass filter and emerge at the third port, to propagate along the waveguide in its original direction. The second signal will again be transmitted by the bandpass filter and emerge via the first port, to propagate along the waveguide in its original opposite direction.
Such a device, however, is limited to a single wavelength in each direction. Increasingly, optical communications systems employ wavelength-division multiplexing (WDM) enabling several signals having different wavelengths to be carried by the same waveguide. A single waveguide might carry any number of different wavelengths in each direction there being, theoretically, no upper limit. Consequently, there is a need for a device which will convert wavelength-division multiplexed signals propagating in a single waveguide into unidirectional signals and then back into bidirectional signals again to continue in their original directions of propagation, and vice versa. There is also a need for apparatus for converting bidirectional WDM optical signals to unidirectional optical signals, or vice versa.